We have already discussed why it could or should be good to calculate metrics on model in a Model Driven Development environment. But we want to argue whether it is a good thing so we should also try to think about reasons why it should not be done.

The UML methodology allows you to express a lot of different kinds of constructs. But if we look at the industry you often see that only a small part of the complete UML language is actually used. The main focus in the industry is on modelling your basic structure. This usually means that only the class diagrams are used to express hwo the software system to be is devided into smaller constructions which can be developed independantly.Sometimes you would also have sequence diagrams or flow diagrams which show how the system is to operate. How you get from state \textbf{X} to state \textbf{Y}. 

This means that the UML diagrams are almost never enough the express the entire system. Thsi makes you think whether it is really realistic to think that you could express the quality of your software system to be in terms of metrics on the architecture. 

There is no doubt that a good architecture helps to improve the quality of your system to be \textbf{\cite{Perry}}, but because the system is almost never completely modelled it could be naive to think that you could express the maintainability of your software system in terms of metrics.